Actual escape area and lateral escape from the xylem of the alkali ions Na+, K+, Rb+ and Cs+ in tomato

Approximation of the total escape area of the xylem in an inbred line of tomato (Ly-copersicon escutentum Mill. cv. Tiny Tim) with help of the frequency distribution of xylem vessel radii provides the possibility to calculate realistic escape constant values from uptake experiments of several elements into tomato stem segments. Comparison of the lateral escape rates of ²⁴Na⁺, ⁴²K⁺, ⁸⁶Rb⁺ and ¹³⁴Cs⁺ indicate that Na⁺ escape is rate-limited by its uptake into a rather constant number of surrounding cells, regardless of changes in the total escape area of the xylem vessels. The escape of K⁺, Rb⁺ and Cs⁺ seems to be proportional to the surface area of the xylem vessels and their escape is apparently controlled by their transport across the cell walls of the transport channels. The calculated small values for the escape rate constants (apparent permeability of the xylem cell walls, ca 2–3 · 10−⁹ m s−⁷) are probably due to the presence of lignin in the xylem cell walls, the discrimination between ions as a result of differing affinities and selectivities and the presence of other solutes in the applied solution.     

Puccinellia tenuiflora maintains a low Na+ level under salinity by limiting unidirectional Na+ influx resulting in a high selectivity for K+ over Na+

Puccinellia tenuiflora is a useful monocotyledonous halophyte that might be used for improving salt tolerance of cereals. This current work has shown that P. tenuiflora has stronger selectivity for K⁺ over Na⁺ allowing it to maintain significantly lower tissue Na⁺ and higher K⁺ concentration than that of wheat under short- or long-term NaCl treatments. To assess the relative contribution of Na⁺ efflux and influx to net Na⁺ accumulation, unidirectional ²²Na⁺ fluxes in roots were carried out. It was firstly found that unidirectional ²²Na⁺ influx into root of P. tenuiflora was significantly lower (by 31–37%) than in wheat under 100 and 150 m m NaCl. P. tenuiflora had lower unidirectional Na⁺ efflux than wheat; the ratio of efflux to influx was similar between the two species. Leaf secretion of P. tenuiflora was also estimated, and found the loss of Na⁺ content from leaves to account for only 0.0006% of the whole plant Na⁺ content over 33 d of NaCl treatments. Therefore, it is proposed that neither unidirectional Na⁺ efflux of roots nor salt secretion by leaves, but restricting unidirectional Na⁺ influx into roots with a strong selectivity for K⁺ over Na⁺ seems likely to contribute to the salt tolerance of P. tenuiflora .     

Promoting effect of fusicoccin on Na+ efflux in barley roots: evidence for a Na+−H+ antiport

Abstract. The effect of fusicoccin (FC) on the K⁺stimulated Na⁺ efflux in root cells of Na⁺ loaded barley roots was studied. FC (0.02 mM) stimulated Na⁺ efflux in the presence of K⁺ and its effect was synergistic with that of K⁺, in a similar way as its effect on proton extrusion. Decreasing the pH of the elution medium promoted Na⁺ efflux and partially replaced the effect of FC. As FC is known to increase the electrochemical proton gradient at the plasmalemma level, these results are consistent with the hypothesis that Na⁺ is extruded in exchange for H⁺. A further support to this view came from the finding that Na⁺ efflux was also promoted by a lipophilic cation, tributylbenzylammonium (TBBA ⁺), which stimulates H ⁺ extrusion and is generally accepted not to enter the cells by means of the same carrier as K ⁺.     

Hypoxia Enhances [3H]Noradrenaline Release Evoked by Nicotinic Receptor Activation from the Human Neuroblastoma SH-SY5Y

Abstract: We have used the human sympathetic neuronal line SH-SY5Y to investigate the effects of hypoxia on noradrenaline (NA) release evoked by either raised [K⁺]_o (100 m M ) or the nicotinic acetylcholine receptor (nAChR) agonist dimethylphenylpiperazinium iodide (DMPP). NA release was monitored by loading cells with [³H]NA and collecting effluent fractions from perfused cells kept in a sealed perifusion chamber. Cells were challenged twice with either stimulus and release was expressed as that evoked by the second challenge as a fraction of that evoked by the first. K⁺-evoked release was unaffected by hypoxia (P o ₂≅ 30–38 mm Hg), but release evoked by DMPP was significantly increased. For both stimuli, replacement of Ca²⁺_o with 1 m M EGTA abolished NA release. K⁺-evoked release was also dramatically reduced in the presence of 200 µ M Cd²⁺ to block voltage-gated Ca²⁺ channels, but DMPP-evoked release was less affected. In hypoxia, DMPP-evoked Cd²⁺-resistant NA release was dramatically increased. Our findings indicate that hypoxia increases NA release evoked from SH-SY5Y cells in response to nAChR activation by increasing Ca²⁺ influx through the nAChR pore, or by activating an unidentified Cd²⁺-resistant Ca²⁺-influx pathway. As acetylcholine is the endogenous transmitter at sympathetic ganglia, these findings may have important implications for sympathetic activity under hypoxic conditions.     

Mechanisms of cobalt uptake in plants: 60CO uptake and distribution in Chara

The mechanism of cobalt uptake was investigated using cells of the giant alga Chara corallina in which it is possible to resolve separately uptake by the cell wall and actual influx across the cell membrane. The absorption of ⁶⁰Co by Chara cells appeared to saturate within 2 h, but this was mainly due to rapid uptake into the cell wall which accounted for 87–92% of the total activity. Even after prolonged desorption most of the cell‐associated ⁶⁰Co was found on the cell wall. The intracellular distribution of absorbed ⁶⁰Co was investigated by fractionating the cell into cytoplasm and vacuole. It was shown that ⁶⁰Co influx to the vacuole occurs simultaneously with influx to the cytoplasm. The transported species appears to be Co²⁺ rather than the less charged Co(OH)⁺ or Co(OH)₂. ⁶⁰Co influx is pH dependent (optimum pH 7–9), and is sensitive to some other divalent metals. Influx from solutions containing 1 µ M ⁶⁰Co was inhibited by 5 µ M Cd²⁺, Cu²⁺, and Zn²⁺, but Mn²⁺ and Ni²⁺ had no significant effect. The sensitivity of Co uptake to N ‐ethyl maleimide (NEM) and cysteine suggests that the transport system involves direct binding of CO²⁺ to ‐SH groups.     

Cations Affect [3H]Mazindol and [3H]WIN 35,428 Binding to the Human Dopamine Transporter in a Similar Fashion

Abstract: The present study addresses the possibility that there are different cocaine-related and mazindol-related binding domains on the dopamine transporter (DAT) that show differential sensitivity to cations. The effects of Zn²⁺, Mg²⁺, Hg²⁺, Li⁺, K⁺, and Na⁺ were assessed on the binding of [³H]mazindol and [³H]WIN 35,428 to the human (h) DAT expressed in C6 glioma cells under identical conditions for intact cell and membrane assays. The latter were performed at both 0 and 21°C. Zn²⁺ (30–100 µ M ) stimulated binding of both radioligands to membranes, with a relatively smaller effect for [³H]mazindol; Mg²⁺ (0.1–100 µ M ) had no effect; Hg²⁺ at ∼3 µ M stimulated binding to membranes, with a relatively smaller effect for [³H]mazindol than [³H]WIN 35,428 at 0°C, and at 30–100 µ M inhibited both intact cell and membrane binding; Li⁺ and K⁺ substitution (30–100 m M ) inhibited binding to membranes more severely than to intact cells; and Na⁺ substitution was strongly stimulatory. With only a few exceptions, the patterns of ion effects were remarkably similar for both radioligands at both 0 and 21°C, suggesting the involvement of common binding domains on the hDAT impacted similarly by cations. Therefore, if there are different binding domains for WIN 35,428 and mazindol, these are not affected differentially by the cations studied in the present experiments, except for the stimulatory effect of Zn²⁺ at 0 and 21°C and Hg²⁺ at 0°C.     

Cation-stimulated H+ efflux by intact roots of barley

Abstract. Rates of proton extrusion and potassium (⁸⁶Rb) influx by intact roots of barley ( Hordeum vulgare cvs . Fergus, Conquest and Betzes) plants were simultaneously measured in short-term (15min) experiments. The nature and extent of apparent coupling between these ion fluxes was explored by manipulating conditions of temperature, pH and cation composition and concentration during flux determinations. In addition, the influence of salt status upon these fluxes was examined. At low K⁺ concentrations (0.01 to 1 mol m⁻³), H⁺ efflux and K⁺ influx were strongly correlated in both low- and high-K⁺ roots, although K⁺: H⁺ exchange stoichiometries were almost consistently greater than 2:1. At higher concentrations (1 to 5 mol m⁻³), H⁺ efflux was either reduced or remained unchanged while K⁺ influxes increased. In the presence of Na₂SO₄, rates of H⁺ extrusion demonstrated similar cation dependence, although below 10 mol m⁻³ Na₂SO₄, H⁺ fluxes were generally 50% lower than in equivalent concentrations of K₂SO₄. These observations are considered in the context of current hypotheses regarding the mechanisms of k⁺/H⁺ exchange.     

Modulation of 45Ca2+ Influx into Cells Stably Expressing Recombinant Human NMDA Receptors by Ligands Acting at Distinct Recognition Sites

Abstract: A ⁴⁵Ca²⁺ influx assay has been used to investigate the pharmacology of stably expressed recombinant human NR1a/NR2A and NR1a/NR2B N -methyl- d -aspartate (NMDA) receptors. Inhibition of glutamate-stimulated ⁴⁵Ca²⁺ influx by six glycine-site antagonists and inhibition of glycine-stimulated ⁴⁵Ca²⁺ influx by five glutamate-site antagonists revealed no significant differences between affinity values obtained for NR1a/NR2A and NR1a/NR2B receptors. The polyamine site agonist spermine showed differential modulation of glutamate- and glycine-stimulated ⁴⁵Ca²⁺ influx for recombinant NMDA receptors, inhibiting and stimulating ⁴⁵Ca²⁺ influx into cells expressing NR1a/NR2A receptors (IC₅₀ = 408 µ M ) and NR1a/NR2B receptors (EC₅₀ = 37.3 µ M ), respectively. The antagonist ifenprodil was selective for NR1a/NR2B receptors (IC₅₀ = 0.099 µ M ) compared with NR1a/NR2A receptors (IC₅₀ = 164 µ M ). The effects of putative polyamine site antagonists, redox agents, ethanol, and Mg²⁺ and Zn²⁺ ions were also compared between NR1a/NR2A and NR1a/NR2B receptors. This study demonstrates the use of ⁴⁵Ca²⁺ influx as a method for investigating the pharmacology of the numerous modulatory sites that regulate the function of recombinant human NMDA receptors stably expressed in L(tk-) cells.     

Effect of ultraviolet radiation on accumulation and leakage of 86Rb+ in guard cells of Vicia faba

The effects of UV-C (254 nm), UV-A (365 nm) and broad-band UV (280–380 nm) on guard cells of Vicia faba L. cv. Long Pod were investigated in the presence of white light (450 μmol m⁻² s⁻¹). UV-C (7 μmol m⁻² s⁻¹) was found to cause leakage of ⁸⁶Rb⁺ from guard cells, while UV-A (0.3 μmol m⁻² s⁻¹) stimulated increased uptake in these cells. A relatively small stimulatory effect was observed by broad-band UV (3 μmol m⁻² s⁻¹) during the first 30 min of irradiation with an apparent equilibration of influx and efflux thereafter. Leakage of ⁸⁶Rb⁺ from guard cells continued despite the removal of UV-C and an increase in the amount of white light from 450 to 1500 μmol m⁻² s⁻¹, suggesting that membranes were irreversibly damaged. Irradiation of guard cells with UV-C for 30, 45 and 90 min indicated that these cells began to be affected already by 30 min UV-C irradiation.     

Elevated Levels of Glucose and L-Fucose Reduce 22Na+ Uptake and Whole Cell Na+ Current in Cultured Neuroblastoma Cells

Abstract: Na⁺ flux was studied in cultured neuroblastoma cells grown in medium containing increased glucose or L - fucose concentrations. Chronic exposure of neuroblastoma cells to 30 m M glucose or 30 m M L-fucose caused a decrease in ouabain-sensitive and veratridine-stimulated ²²Na⁺ uptake compared with cells cultured in unsupplemented medium. The Na⁺ current, determined by using whole-cell configuration of the patch clamp, was also decreased in these cells. Tetrodotoxin (3 μ M ), which blocked whole cell Na⁺ currents, also blocked veratridine-stimulated ²²Na⁺ accumulation. Culturing cells in medium containing 30 m M fructose as an osmotic control had no effect on Na⁺ flux. Specific [³H] saxitoxin binding was not affected by 30 m M glucose or 30 m M L-fucose compared with cells grown in unsupplemented medium, suggesting that the number of Na⁺ channels was not decreased. These studies suggest that exposing cultured neuronal cells to conditions that occur in the diabetic milieu alters Na⁺ transport and Na⁺-channel activity.     

Studies of Inositol Phosphate Export from Neuronal Tissue In Vitro

Abstract: Recent in vivo microdialysis studies have demonstrated the presence of extracellular levels of inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] that can be increased in a concentration-dependent manner by muscarinic receptor activation. The aim of the present study was to determine whether extracellular levels of Ins(1,4,5)P₃ could be measured in vitro. Despite rapid increases in internal Ins(1,4,5)P₃ levels after stimulation with 1 m M carbachol, there was no change in external levels in both rat brain cortical slices and human neuroblastoma SH-SY5Y cells. Suprafusion of myo -[³H]inositol-prelabelled hippocampal slices with 1 m M carbachol caused an increase in ³H-inositol phosphates over basal levels in the perfusate after 10 min, reaching a peak (223 ± 56% of basal) 20 min after suprafusion with carbachol was started. This response to carbachol was potentiated in the presence of 30 m M K⁺. Analysis of the individual ³H-inositol phosphates in the perfusate revealed that levels of [³H]inositol monophosphate, [³H]inositol bisphosphate, [³H]inositol trisphosphate, and [³H]inositol tetrakisphosphate were all significantly increased. A similar increase in extracellular ³H-inositol phosphates was demonstrated in SH-SY5Y cells incubated with 1 m M carbachol for 30 min. This response was again enhanced by 30 m M K⁺, although the intracellular response was not potentiated. Possible roles for extracellular inositol phosphates are discussed.     

Growth and accumulation of N, K+, Ca2+ and Mg2+ in barley exposed to various nutrient regimes and root/shoot temperatures

Six cultivars of spring barley ( Hordeum vulgare L. cvs Salve, Nümberg II, Bomi, Risø 1508, Mona and Sv 73 608) were grown in water culture for three weeks with various combinations of mineral supply and differential roots/shoot temperatures during the growth period. Most important for growth and accumulation of N, K⁺, Ca²⁺ and Mg²⁺ was the mineral supply, followed by the root temperature and the choice of cultivar. Treatments with low mineral supply or low root temperature induced a uniform reduction in growth and accumulation of the ions studied. The effects of low mineral supply and low root temperature on growth and N accumulation was additive, which indicates that these factors exert their influence independently of each other.
Roots grown at 10°C were smaller and Rb⁺(⁸⁶Rb) influx was higher than in roots grown at 20°C. It is suggested that the control of Rb⁺(⁸⁶Rb) influx is affected by the root temperature and the age of the plants. The higher ⁸⁶Rb⁺ (⁸⁶Rb) influx into the low temperature roots could not compensate for the smaller root size. However, the lower total mineral accumulation made up for the needs of the smaller plants and cannot explain the reduction in growth.     

Ca2+ dependence and La3+ interference of ultraviolet radiationinduced K+ efflux from rose cells

A low fluence of ultraviolet radiation (UV) causes cultured cells of Rosa damascena Mill cv. Gloire de Guilan to lose intracellular K⁺. This effect required the presence of Ca²⁺ in the medium. A reduction in the concentration of free Ca²⁺ to 10⁻⁵ M with ethyleneglycol-bis-(β-aminoethyl-ether)-N.N.N',N'-tetraacetic acid (EGTA) buffer inhibited the UV-stimulated efflux; this was correlated with a discharge of the membrane potential and a stimulation of the leakage of K⁺ from unirradiated cells. All the same effects were seen with La³⁺ at 0.2 m M. At 0.02 m M La³⁺, the UV-stimulated efflux of K⁺ was blocked without concomitant effects on the membrane potential or K⁺ efflux from control cells. It is suggested that removal of Ca²⁺ blocks or masks the UV-induced leakage of K⁺ by destabilizing the plasma membrane. In addition, La³⁺ may specifically inhibit the UV-stimulated opening of K⁺ or anion channels.     

ENERGETICS OF AMINO ACID TRANSPORT INTO BRAIN SLICES: EFFECTS OF K+ DEPLETION AND Rb+ OR Cs+ SUBSTITUTION ON AMINO ACID UPTAKE

Abstract— Mouse brain slices were depleted of K⁺ by three 10-min incubations-in oxygenated HEPES-buffered medium lacking glucose and K⁺. Addition of K⁺ or Rb⁺ (or Cs⁺, to a smaller degree) with glucose, or with succinate, malate, and pyruvate (SMP) before incubation at 37°C with ¹⁴C-amino acids restored active low-affinity transport of d -Glu, α-aminoisobutyrate (AIB), GABA, Gly, His, Val, Leu, Lys, and Orn. Ouabain at 1–2μ m with Rb⁺ was more inhibitory with SMP than with glucose, suggesting that the glycoside may affect specific energy coupling to transport. Valinomycin, in contrast, showed no specificity of inhibition of amino acid uptake with glucose or SMP and K⁺ or Rb⁺. Cs⁺ partially restored amino acid uptake, but Li⁺ was less effective than Cs ⁺. NaF at 10 m m with SMP + Rb⁺, or SMP + K⁺ did not inhibit amino acid uptake. Therefore, it was possible to dissociate glycolysis and Na⁺, K ⁺ -ATPase activity from amino acid transport. The ion replacements for K ⁺ that supported active amino acid transport indicate that the specificity of ions in possible ionic gradients for transport energetics should be reexamined.     

Growth, contents of K+ and kinetics of K+(86Rb) uptake in barley cultured at different low supply rates of potassium

Plants of barley ( Hordeum vulgare L. cv. Salve) were grown with 6.5–35% relative increase of K⁺ supply per day (RKR) using a special computer-controlled culture unit. After a few days on the culture solution the plants adapted their relative growth rate (RGR) to the rate of nutrient supply. The roots of the plants remained in a low salt status irrespective of the rate of nutrient supply, whereas the concentration of K⁺ in shoots increased with RKR. Both V_max and K_m for K⁺(⁸⁶Rb) influx increased with RKR. It is concluded that with a continuous and stable K⁺ stress, the K⁺ uptake system is adjusted to provide an effective K⁺ uptake at each given RKR. Allosteric regulation of K⁺ influx does not occur and efflux of K⁺ is very small.     

Synthesis of Proteins Enriched in Li+- Induced Vegetalized Embryos of Sea Urchin during Early Development

Sea urchin embryos were vegetalized by a pulse treatment with 60 mM Li⁺ between 2.5 hr and 6 hr after fertilization at 20°C. Normal and Li⁺ -treated embryos were exposed to [³⁵S]-methionine for 2 hr at various stages and [³⁵S]-labeled proteins were analyzed by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). On fluorograph of 2D-PAGE at the pre-hatching blastula stage, significant difference of labeled proteins between normal and vegetalized embryos was not observed in the range from neutral to acidic pH, but pls of several proteins were found to be shifted toward alkaline pH. At the mesechyme blastula stage, five major proteins [M.W. 36 K, 43 K (two species), 71 K and 150 K] were enriched in Li⁺-treated embryos among a few hundreds of synthesized proteins. At the late gastrula stage, the labeling intensities of these proteins except for one of 43 K proteins increased remarkably in Li⁺-treated embryos. Furthermore, two proteins (M.W. 105 K and 135 K) were also enriched in Li⁺-treated embryos at this stage. At the prism stage, these proteins enriched in Li⁺-treated embryos became hardly detectable, and the synthesis of at least four proteins (M.W. about 20 K, 41 K, 43 K and 200 K<) appeared to increase in normal embryos, but not in Li⁺-treated embryos. Synthesis of proteins eniched in Li⁺-treated embryos probably support endodermal cell differentiation.     

Adenosine A2a Receptor-Mediated Modulation of Striatal [3H]GABA and [3H]Acetylcholine Release

Abstract: The ability of adenosine agonists to modulate K⁺-evoked 4D†-[³H]aminobutyric acid ([³H]GABA) and acetylcholine (ACh) release from rat striatal synaptosomes was investigated. The A_2a receptor-selective agonist CGS 21680 inhibited Ca²⁺-dependent [³H]GABA release evoked by 15 m M KCI with a maximal inhibition of 29 ± 4% (IC₅₀ of ∼4 ± 10 ⁻¹² M ). The relative order of potency of three agonists was CGS 21680 ± 5'- N -ethylcarboxamidoadenosine > R-phenylisopropyladenosine (R-PIA), with the inhibition being blocked by A_2a receptor-selective antagonists (CP 66,713 and CGS 15943A) but not by the A₁-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). When release of [³H]GABA was evoked by 30 mM KCI, no significant inhibition was observed. In contrast, CGS 21680 stimulated the release of [³H]ACh evoked by 30 m M KCI, with a maximal stimulation of 26 ± 5% (IC₅₀ of ∼10⁻¹¹ M ). This effect was blocked by CP 66,713 but not by DPCPX. The A₁ agonist R -PIA inhibited [³H]ACh release, an effect blocked by DPCPX. It is concluded that adenosine A_2a receptors are present on both GABAergic and cholinergic striatal nerve terminals where they inhibit and stimulate transmitter release, respectively. Key Words : GABA—Acetylcholine—Adenosine receptors—Striatum.     

RELEASE OF [3H]GAMMA-AMINOBUTYRIC ACID FROM GLIAL CELLS IN RAT DORSAL ROOT GANGLIA.

Abstract— Desheathed rat dorsal root ganglia were incubated in a medium containing amino-oxyacetic acid and [³H]GABA. Under these conditions, [³H]GABA is taken up exclusively by the satellite glial cells in the ganglia. Efflux of [³H]GABA from the tissue was measured after passing the ganglia through a series of wash solutions. The spontaneous efflux of radioactivity, mostly [³H]GABA, was more rapid in the absence of amino-oxyacetic acid in the incubation and wash media.
Raising the potassium concentration in the wash media caused an increase in the efflux of [³H]GABA. This increase was sigmoidally related to the potassium concentration in the wash media, reaching a maximum at 64 m m -K⁺. The releasing effect of K⁺ was inhibited by removing calcium from the media. Reducing the calcium and raising the magnesium concentration in the wash solutions inhibited the increased efflux of [³H]GABA due to 64 m m -K⁺ by 48 per cent, while 5 mM-La³⁺ and diphenylhydantoin (0·005 and 0·5 m m ) had no effect on this increase.
Only a small increase in the efflux of [¹⁴C]glutamate was produced by 64 m m -K⁺ and it had no effect upon the effluxes of [³H]glycine, [³H]alanine or [³H]leucine. The efflux of lactate dehydrogenase was similarly unaffected by 64 mM-K⁺. The results suggest that glial cells in spinal ganglia can respond to depolarizing concentrations of potassium by releasing GABA in a calcium-dependent process.     

Excretion of ions (Cd2+, Li+, Na+ and Cl−) by Tamarix aphylla

Excretion of minerals by the NaCl-resistant and comparatively cadmium-resistant tree Tamarix aphylla (L.) Karst, was investigated. Cd²⁺ was excreted by plants exposed for 1–10 days to 9 or 45 μ M Cd²⁺ solutions. Excretion of this toxic ion increased considerably with time but was less than 5% of the quantities that had been accumulated in the shoots. Excretion of Na⁺ and Cl⁻ was positively correlated with NaCl concentration (1.5, 10, 50 m M ) of the medium. The Na⁺/Cl⁻ ratios of the excrete were positively correlated with the concentration of the treatment solution. Ca²⁺ excretion decreased with increasing NaCl concentrations of the solution. Excretion of K⁺ and Mg²⁺ was only little affected by NaCl. Excretion of Li⁺ occurred whenever this element was supplied in the uptake solution; daily excretion rates of Li⁺ increased with time. The ecological significance of excretion is discussed in relation to the low selectivity of the mechanism in T. aphylla .     

Blockade by Neurotransmitter Antagonists of Veratridine-Activated Ion Channels in Neuronal Cell Lines

Abstract: The voltage-dependent Na⁺ ionophore of various neuronal cells is permeable not only to Na⁺ ions but also to guanidinium ions. Therefore, the veratridine-(or aconitine-) stimulated influx of [¹⁴C]guanidinium in neuroblastoma × glioma hybrid cells was measured to characterize the Na⁺ ionophore of these cells. Half-maximal stimulation of guanidinium uptake was seen at 30 μ M veratridine. At 1 m M guanidinium, the veratridine-stimulated uptake of guanidinium was lowered to 50% by approximately 60 m M Li⁺, Na⁺, or K⁺ and by a few millimolar Mn²⁺, Co²⁺, or Ni²⁺. The basal, as well as the veratridine-stimulated, uptake of guanidinium was inhibited by the cholinergic antagonists (+)-tubocurarine ( K_i = 50 to 500 n M ) and atropine ( K_i = 5 to 30 μ M ) and the adrenergic antagonists phentolamine ( K_i = 5 μ M ) and propranolol ( K_i = 60 μ M ). The specificity of the inhibitory effects of these agents is stressed by the ineffectiveness of various other neurotransmitter antagonists. However, the corresponding ionophore in neuroblastoma cells (clone N1E-115) seems to be regulated differently. While phentolamine and propranolol inhibit the veratridine-activated uptake as in the hybrid cells, (+)-tubocurarine and atropine exert only a slight effect.